Hi all, I am running Windows 2003 Server R2 SP1 and Exchange Server 2003 SP2, and have been since the start of November '06 without any issues, along with a WatchGuard router. Over the past two weeks I have suddenly had various clients ringing and saying that email to us is being returned and pointing them to the SPF error. These are clients that have been emailing us for 10 years in some cases and most use different ISP's. Nothing has changed at our end (no updates etc.. in that time). Our MX record points to a 3rd party AntiVirus/AntiSpam vendor and then forwards to our external IP. I have spoken to both that 3rd party vendor and also our ISP provider who assure me that they haven't turned on SPF filtering although I believe the issue must lay with one of them. Am I missing something?